Compared to charge coupled device (CCD) sensors, a Complementary Metal-Oxide-Semiconductor (CMOS) image sensor has many advantages such as low voltage operation, low power consumption, compatibility with logic circuitry, random access, and low cost. Among the CMOS image sensors, a phase detection auto-focus (PDAF) CMOS image sensor is widely used because the PDAF CMOS image sensor provides excellent auto focus function. In the PDAF CMOS image sensor, some photodiodes are partially shielded by metallic grid to provide phase detection function. The photodiodes partially shielded by metallic grid will result in lower incident signal when pixel size of the PDAF CMOS image sensor shrinks. Accordingly, the performance of the PDAF CMOS image sensor is compromised.